The training room or gym or whatever it may I think of it as a room that can be used to level up the NPCs in the dungeon instead of waiting for invaders to arrive.  

I don't know if there's a thread that already has this kind of idea mentioned but a training room could be useful.

not shooting you down, or anything, but with the altar we can summon heroes at any time (so long as we can afford it,)  and you can also level NPCs by sending them on missions to the overworld.

Right, thanks.